Wireless communication devices require a battery or external DC power supply for a power source. Within a wireless communication device, there are integrated circuits (ICs). These ICs typically operate at a much lower DC voltage than either a battery or an external DC power supply attached to the wireless communication device. To facilitate integrated circuits operation at a low operating voltage, a switching voltage regulator is usually required to convert either an external DC power supply or battery voltage to the integrated circuits lower supply voltage.
A switching voltage regulator provides the highest power efficiency when the difference between the battery voltage (VBAT) and the integrated circuits supply voltage (VDD) is more than a couple hundred millivolts. In one particular example, the battery is composed of a Li-ion cell with a 3.6V nominal voltage and the integrated circuits operate at 1.8V. Therefore, the difference between the battery voltage and the integrated circuits voltage is 3.6 V-1.8 V or 1.8 V. In this particular example, a switching voltage regulator is strongly preferred over a linear regulator. A linear regulator would experience the full 1.8V drop between the battery and the load. The power dissipated by a linear regulator is equal to 1.8V*IDD (the load current of the integrated circuits). As such, a switching voltage regulator may dissipate only 10% of the energy used by the integrated circuit (over a wide range of load current), whereas a linear regulator would dissipate 100% of the energy used by the integrated circuit regardless of the load current. Switching voltage regulators are often used in wireless communication devices for this reason.
Switching voltage regulators may convert between a higher input voltage and a lower output voltage using one or more electronic switches in conjunction with energy storage devices (inductors or capacitors) to transfer energy between a higher external DC power supply voltage and a lower integrated circuit voltage.
By way of background, the ratio between a switching voltage regulator output voltage (Vo) and a switching voltage regulator input voltage (Vi) sets the duty cycle (D) for a switching voltage regulator (D=Vo/Vi). A switching voltage regulator frequency is dictated by the output voltage ripple requirement, the size of the series inductor and load filtering capacitor within a switching voltage regulator, output DC load current, and desired power efficiency of a switching voltage regulator. In the case where the switching voltage regulator is coupled to other radio frequency (RF) transceiver circuits, a switcher frequency of the switching voltage regulator can cause interference with other such components in the wireless communication device. This interference appears as voltage ripple on the VDD and ground connections of the RF transceiver circuit. This voltage ripple is composed of discrete frequency components. Each frequency component is a harmonic of the switcher frequency of the switching voltage regulator. A power level of each harmonic is dependent on (i) the duty cycle of the switcher frequency of the switching voltage regulator, (ii) the degree of capacitive filtering of the output voltage, as well as (iii) the type of coupling between the switching voltage regulator and the RF transceiver circuit.
Radio frequency (RF) voltage-controlled oscillators (VCOs) are typically embedded in a RF transceiver and function as local oscillator(s) (LOs) to up-convert or down-convert communication signals from/to baseband to/from RF. In a typical configuration with a switching voltage regulator either directly or indirectly coupled to the RF VCO, voltage ripple at the output of the switching voltage regulator may combine with a frequency tuning element voltage of the RF VCO to create frequency modulation (FM) on the RF VCO output at offsets equal to the harmonics of the switching voltage regulator switcher frequency. The switching voltage regulator induced FM on the RF VCO causes harmonic spurious content to appear at offsets from the fundamental output carrier frequency of the RF VCO.
This harmonic spurious content induced by the switching voltage regulator (directly or coupled on to the RF VCO) may interfere with the performance of a wireless communication device under certain operating conditions. For example, weak receive signal strength, the presence of external jammers at specific frequency offsets from a desired receive channel, and/or transmit leakage into a receive path in a full-duplex transceiver can all contribute to greater interference, in the presence of the switching voltage regulator, on the analog signals to be up- or down-converted to or from radio frequencies.
Known ways to reduce the effect of switcher frequency spurious content caused by the switching voltage regulator in a wireless communication devices include: (i) adjusting the frequency of the switching voltage regulator continuously using pulse width modulation, pulse density modulation, or frequency hopping; b) toggling between a switching voltage regulator and a linear regulator during receive only modes of the wireless communication transceiver; and (iii) moving the switching voltage regulator as far away (using shielding and differential signal paths for improved isolation) from sensitive VCO and other components, all of which introduce a level of design complexity or inefficient use of circuit board or integrated circuit area.
There is a need for improved ways to reduce the effect of interference from the switching voltage regulator in wireless communication devices without the drawbacks of conventional techniques.